CodingGuidelines: update and clarify command-line conventions

Edit the section which explains how to create a good SYNOPSIS section for clarity and accuracy, it was mostly introduced in c455bd8950e (CodingGuidelines: Add a section on writing documentation, 2010-11-04): * Change "extra" example to "file", which now naturally follows from previous "<file>..." example (one or more) to "[<file>...]" (zero or more). * Explain how we prefer spacing around "[]()" tokens and "|" alternatives, this is not a new policy, but just codifies what's already the pattern in the most wide use in the documentation. Having a space around " | " for flags, but not for flag values is inconsistent, but this style guide codifies existing patterns. Grepping shows that we don't have any instance matching the second "Don't" example: git grep -E -h -o '=\([^)]+\)' -- builtin Documentation/ Signed-off-by: Ævar Arnfjörð Bjarmason <avarab@gmail.com> Signed-off-by: Junio C Hamano <gitster@pobox.com>